Various types of developing devices for use in a copier, facsimile, printer or the like are conventionally proposed (reference to JP2000-194194A, JP2000-194195A and JP2000-250311A). FIG. 6 shows a conventional developing device. FIG. 7 is an explanation view showing distribution of developer in a developer tank disposed on a side close to a developer carrier provided in the conventional developing device. FIG. 8 shows oblique unevenness formed by a transferring screw. As shown in FIG. 6, a developing device 220 includes a developer carrier 204 which transfers developer 210 to a developing area opposed to an image carrier 211, and develops an electrostatic latent image formed on the image carrier 211 to obtain a toner image. This developer carrier 204 includes a developing sleeve 203 including a rotatable nonmagnetic cylindrical body 201 and a magnet roller 202 which is disposed inside the developing sleeve 203, and has a plurality of fixed magnetic poles forming magnetic fields on its surface, so as to nap the developer 210 on the surface of developing sleeve 203. When napping the developer 210, carriers including the developer 210 are napped on the developing sleeve 203 along magnetic lines produced by the magnet roller 202, and the charging toner is adhered onto the carriers. The magnet roller 202 includes a plurality of magnetic poles. A magnet forming each of the magnetic poles is formed in, for example, a rod-like compact. Especially, the magnet roller 202 includes a developing major magnetic pole, which naps the developer 210, in a portion corresponding to a developing area portion of the surface of developing sleeve 203. The napped developer 210 can be moved in the circumferential direction by rotating at least one of the developing sleeve 203 and the magnet roller 202. In general, the surface of developing sleeve 203 is appropriately roughened by a sandblast in order to easily transfer the developer 210. Such roughening is mainly performed especially for a color copier and printer. A roughening process such as a groove process or sandblast process is performed to the surface of developer carrier 204, i.e., the surface of developing sleeve 203 in an image forming apparatus of an electrophotography type such as a copier, printer or facsimile, except in the case of low speed. This roughening process such as a groove process, sandblast process is conducted for preventing decrease in image concentration caused by the retention of slipped developer 210 on the surface of developing sleeve 203 which rotates at high speed.
However, in the developing device 220 having an axially even shaft diameter, since a transferring path of the developer 210 which draws the developer 210 moved in the developing tank 208 by the developing sleeve 203 to transfer the developer to the developing area, and separates the developer 210 after passing the developing area on the developer tank 209 side, the balance of developer 210 in the developing device 220 is disturbed over time. For this reason, the developer bulk in the vicinity of the center portion of the first developer transferring screw 206 is reduced, and the peak of developer bulk is formed in the end portion of driven side of the first developer transferring screw 206, as shown in FIG. 7. Therefore, the amount of developer 210 in the vicinity of the center portion of first developer transferring screw 206 is reduced, resulting in a trouble of drawing up the developer 210 onto the developing sleeve 203. Thus, the oblique unevenness is caused as shown in FIG. 8 and also the image concentration is decreased.
Moreover, the developing device 220 has the following problems. (1) Since the first developer transferring screw 206 includes a single blade, it is susceptible to the developer bulk, causing significant oblique unevenness onto an image. (2) If the developer 210 is used for a long period of time, the surface of carrier is filled by addition agent, the surface film of carrier abrades away or the like. For this reason, the power characteristic of developer 210 is changed; thus, the amount of developer which is drawn up onto the developing sleeve 203 is changed. (3) After performing a cutting process or grinding process to the developing sleeve 203 for improving the accuracy of runout, if a sandblast process about 10 μm roughness is conducted to the developing sleeve 203, the transferring performance of the developing sleeve 203 is decreased over time, a high quality image is hardly maintained over a long period of time, combined with the decrease in the developer bulk in the vicinity of the center portion of the first developer transferring screw 206. (4) In the developer 203 having the grooves on the surface, the problem of decrease in the drawing-up over the time, etc., is not serious, but the runout is deteriorated by the stress when performing the process having the grooves. Therefore, it is difficult to obtain the runout accuracy higher than that of the developing sleeve obtained by the sandblast.
Moreover, it is considered to perform a cutting process or grinding process to the surface of developing sleeve 203 after conducting a groove process. However, if the cutting process or grinding process is performed to the surface of the developing sleeve 203 after conducting the groove process to the surface of the developing sleeve 203, causing burry in the groove portion when conducting the cutting process or the grinding process. Accordingly, an image is defected by the separation of burry when using the developing sleeve 203 continuously and also transferring performance is deteriorated.
Furthermore, since the transferring path, which draws the developer 210 from the first developer tank 208 to the developing sleeve 203 to transfer the developer to the developing area opposed to the image carrier 211 and separates the developer 210 after passing the developing area to the second developer tank 209, is formed in the developing device 220, the developer 210 which is transferred to the second developer tank 209 from the first developer tank 208 continues to increase or conversely, i.e., the developer 210 which is transferred to the first developer tank 208 from the second developer tank 209 via second developer screw 207 continues to increase, resulting in the unbalance of developer 210 in the developing device. Therefore, the amount of developer 210 of the center portion of the first developer transferring screw 206 is reduced, and the drawing-up of developer 210 to the developing sleeve 203 is disturbed; thus, an image is deteriorated by the generation of oblique unevenness.
In addition, a technique for disposing a control member 205 above the developing member 203 is proposed (reference to JP2005-181896A). This technique includes a space problem and a problem of complicated processing steps.